In this thesis, ballistic transport and thermoelectric (TE) properties of semiconducting and dynamically stable two-dimensional materials are investigated by combining first-principles calculations with Landauer formalism. Motivated by finding novel promising TE materials, transition metal dichalcogenides (TMDs) and oxides (TMOs) (namely MX2 with M = Cr, Mo, W, Ti, Zr, Hf; X = O, S, Se, Te) are studied systematically in their 2H- and 1T-phases in Chapter 3. Having computed structural, as well as ballistic electronic and phononic transport properties for all structures, we analyze the thermoelectric properties of the semiconducting ones. We report for the first time that, 2H-phases of four of the studied structures have very promising thermoelectric properties, unlike their 1T-phases. Next, ballistic transport and thermoelectric (TE) properties of group IIImonochalcogenides (group III-VI) are presented in a wide range temperature from 100 K to 1000 K. This large family composed of 25 compounds which stands out with their unique electronic band structures. In addition to Mexican hat shaped (quartic energy-momentum relation) valence band character, some of the structures exhibit valley degeneracies which can occur either in their conduction and valence bands. Moreover, TE and transport calculations are performed for BO and BS monolayers which consist of lightest species in group III-monochalcogenides. Surprisingly, BO and BS monolayers exhibit high TE efficiency at low temperatures. Low thermal conductance at low temperatures and stepwise electronic transmission at the valence band edge are the physical mechanisms behind achieving large ZT.